1. Field of the Invention
The present invention relates generally to detection of arterial stenosis and more particularly to a high-speed method and apparatus for screening for arterial stenosis in which a Doppler ultrasound system automatically detects parameters indicative of stenosis in the velocity profile of blood flow through the length of an artery without analyzing visual imagery.
2. Description of the Related Art
Arteriosclerosis is a chronic disease characterized by abnormal thickening and hardening of the arterial walls. From a medical point of view, arteriosclerosis affecting the coronary arteries is of most concern. Many coronary heart diseases, including arteriosclerosis, angina, myocardial infarction (MI), and sudden death, depend in a large part on the severity and distribution of obstructive coronary lesions that develop slowly over a period of years and that lead to stenotic arteries. As a result, periodic screening of patients to detect and assess obstructive coronary lesions is vital for the diagnosis, treatment and prevention of coronary disease.
It is well known that blood flow through a segment of healthy artery has a parabolic shaped velocity profile. Upon entering a narrowed segment of an artery, however, blood flow accelerates, thereby increasing the momentum and the kinetic energy of the flow. Substantial energy loss occurs at the exit of the narrow segment of the artery, causing turbulence and vortices at the boundaries of the artery. These phenomena increase with an increase in the magnitude of the stenosis and in the magnitude of normal flow velocity through the artery.
The velocity of blood flow of a person at rest is typically in the range of 30 to 50 cm/sec in healthy coronary arteries with a diameter ranging between 1 to 3 mm. In general, blood flow of a person at rest begins to be affected with an 80 to 90 percent reduction in diameter. For a person experiencing maximum blood flow, typically during strenuous exercise, blood flow may be affected with only a 45 percent reduction in diameter.
One imaging method for direct detection of coronary stenosis is coronary arteriography. This is an expensive invasive imaging procedure that is not practical for routine screening. Other non-invasive imaging methods for performing cardiac screening include computerized axial tomography (CAT) and magnetic resonance imaging (NMI). These procedures use still framed images to determine whether a particular section of artery appears to be obstructed. While non-invasive, these procedures are costly and typically cannot be performed as part of a regular check up.
More recently, ultrasound imaging systems have been employed to detect and measure stenosis in the carotid artery by imaging the blood flow in the artery. Presently available ultrasound systems utilize the Doppler principle. In traditional Doppler ultrasound systems, a transducer directs a beam of ultrasonic energy toward a blood vessel in which blood flow information is desired. Moving blood cells reflect the ultrasound energy, called echoes, and either increase or decrease the frequency of the reflected energy depending on the direction of blood flow and the angle of incidence of the beam. In continuous wave systems, a second transducer receives the echo and detects the frequency shift from which velocity of the blood flow may be calculated. In pulse wave systems, a single transducer is used to direct the beam and receive the echo with a filter sorting out the signals to determine the frequency shift and hence the velocity of blood flow.
One limitation of Doppler ultrasound technology is that such systems can only measure the projection of the velocity flowing in the direction of the beam. If the ultrasound beam is directed perpendicular to the direction of flow, no flow will be recorded. If the beam is pointing at some angle with respect to the flow, the recorded velocity will be lower than the actual velocity to a degree proportional to the cosine of the angle. To overcome this limitation, duplex Doppler ultrasound systems, which allow imaging to be used along with traditional Doppler ultrasound systems, are used so that a region of interest may be xe2x80x9ceye-balledxe2x80x9d by an ultrasound technician and the beam may either be positioned at an appropriate angle or the angle of measurement may be recorded.
Another problem with the use of ultrasound systems is the presence of xe2x80x9cnoisexe2x80x9d components in the Doppler shift frequency. The walls of blood vessels are dynamic in that they move in phase with a beating heart. During the systolic portion of the cardiac cycle the walls move out and during the diastolic portion the walls move in. These movements result in low and high frequency noise components returning with the echo of the Doppler signal.
Notwithstanding advances made in the art, all of the present methods and devices cannot in practice be used to periodically screen the heart region for potential stenotic areas. In general, these methods and devices rely upon visual imaging systems, which have difficulty forming images of the complex human anatomy, and require slow human visual analysis of each image. Therefore, there exists a need for a simple inexpensive non-invasive method of screening for arterial stenosis using a Doppler ultrasound system that does not rely upon visual imaging and is relatively unaffected by the shortcomings of previous ultrasound systems.
The present invention is a non-invasive method and apparatus for detecting and measuring the degree of coronary and other arterial stenosis without the need for imaging the artery. In accordance with a first method of the invention, a Doppler ultrasound system scans numerous segments of arteries to construct velocity profiles of blood flow across each segment. The path of the artery is determined based on the topographic location of the arterial flow sections, and the shape and skew of the velocity profiles are measured. A delta velocity profile representing the change in velocity along the length of the mapped artery may also be measured. The velocity profile and delta velocity profile are then analyzed to determine potentially stenotic areas within the artery.
A reference coordinate system is defined on the patient so that the location of the Doppler ultrasound system and potentially stenotic areas may be accurately determined. If desired, a visual Doppler image of potentially stenotic areas of the artery may be displayed a upon detection of stenotic areas or at the end of the scanning process. In a preferred embodiment, velocity profile parameters selected from the group consisting of v, dh, b, n, s, and w (each as described below) or V, DH, B, N, S, individually or in any combination thereof, are compared with predefined threshold values to determine potentially stenotic areas within the artery.
After reconstruction of the arterial paths, changes in the parameters indicated above along the path of the artery are compared with predefined thresholds values to determine potentially stenotic areas. In an alternate embodiment, parameters can be measured in both the diastole and systole stages so that their difference as well as their individual values can be evaluated and compared to predefined thresholds. Scanning by the Doppler ultrasound system may be triggered based on detection of a desired point in time on an ECG of the patient.
The Doppler ultrasound system comprises at least one scanner, and preferably comprises a plurality of scanners. The step of defining a reference coordinate system comprises the step of selecting a reference point on the patient and, if desired, measuring angles of reference between the scanners of the Doppler ultrasound system and the selected reference point. The Doppler ultrasound system may be carried on a robotic arm to facilitate more accurate detection of the position of the ultrasound scanner.
Various parameters of the velocity profile may be analyzed to determine potentially stenotic areas, including:
i) The average velocity (v) of the velocity profile;
ii) The peak change in velocity (dh) across the velocity profile;
iii) The presence and degree of blunt flow (b) found in the velocity profile;
iv) The presence and degree of turbulence (N) found in the velocity profile;
v) The degree of skew (s) in the velocity profile; and
vi) The width (w) of the velocity profile.
After reconstructing the path of the artery based on the individual velocity profiles measured, the change in each of the parameters (v, dh, b, n, s, and w) along the length of the artery is examined to determine potentially stenotic areas. These additional parameters can be defined respectively as: (xcex94v/xcex94L)a, (xcex94dh/xcex94L)a, (xcex94b/xcex94L)a, (xcex94dh/xcex94L)a, (xcex94s/xcex94L)a, and (xcex94w/xcex94L)a where xcex94L is the distance between two consecutive velocity profiles along the path of an artery, and a is the position along the artery.
An alternative method of detecting arterial stenosis comprises: using a Doppler ultrasound system to measure a plurality of parameters indicative of a potentially stenotic artery; determining from the measured parameters the degree of stenosis in the artery; and scoring the degree of stenosis and determining coordinates of the potentially stenotic artery.
A further alternative method of detecting arterial stenosis comprises: detecting a signal representative of the velocity of blood flow in an artery; on-structing a series of velocity profiles of the blood flow in the artery; based on the velocity profiles, reconstructing the path of the artery; and based on the velocity profiles, their coordinates and changes in the velocity profile along the path of the artery, identifying potentially stenotic areas in the artery. The velocity profiles are used to determine parameters selected from the group consisting of v, dh, b, n, s, and w, and one or more of the parameters is compared with predefined threshold values to determine potentially stenotic areas within the artery. The delta velocity profiles are used to determine parameters selected from the group consisting of xcex94v/xcex94L, xcex94dh/xcex94L, xcex94b/xcex94L, xcex94n/xcex94L, xcex94s/xcex94L, and xcex94w/xcex94L, and one or more of the parameters is compared with predefined threshold values to determine potentially stenotic areas within the artery. Changes in these parameters along the length of the artery (xcex94v/xcex94L)a, (xcex94dh/xcex94L)a, (xcex94b/xcex94L)a, (xcex94n/xcex94L)a, (xcex94s/xcex94L)a, and (xcex94w/xcex94L)a may also be compared with thresholds to determine stenotic areas.
Another method of detecting arterial stenosis comprises: detecting a defined point in time on an ECG; upon detection of the defined point, scanning an area of interest using a Doppler ultrasound system to construct a series of velocity profiles of blood flow through the length of the arteries present; reconstructing the path of the arteries; analyzing the velocity profiles; and analyzing delta velocity profiles to determine potentially stenotic areas within the arteries.
An apparatus for detecting stenosis of an artery according to the invention comprises a Doppler ultrasound system for scanning a segment of the artery and for receiving echoes of the Doppler ultrasound signal; and a processor for receiving the echoes and for constructing velocity profiles of blood flow through the area of interest. The processor analyzes the velocity profiles to determine potentially stenotic areas within the artery. The system includes means for defining a reference coordinate system relative to the area of interest, with the processor determining a location of the Doppler ultrasound system in the reference coordinate system upon detecting potentially stenotic areas within the artery. A display is provided for displaying a Doppler image of a potentially stenotic area of the artery upon detection of such stenotic areas.
An alternative apparatus for detecting arterial stenosis includes: a Doppler ultrasound system for measuring a plurality of parameters indicative of a potentially stenotic artery; and a processor for determining from the measured parameters the degree of stenosis in the artery. The processor scores the degree of stenosis and determines the coordinates of the potentially stenotic artery.
A further alternative apparatus for detecting arterial stenosis includes: means for detecting a signal representative of the velocity of blood flow in an artery; means for constructing a velocity profile and/or delta velocity profile of the blood flow in the artery; and means for identifying potentially stenotic areas in the artery based on the velocity profile and/or changes of the velocity profile along the length of an artery.